Is highly uncertain...
Let's start with the amount of carbon that has to be emitted to produce and deliver a kilogram of concrete. According to the Inventory of Carbon and Energy (ICE v1.6a), the embodied energy of concrete averages 2.9 MJ/Kg, but the standard deviation is 8.7 MJ/Kg, and the range in 122 studies worldwide goes from 0.07 to 92 MJ/Kg. So it all depends, and the impact of any concrete project is highly variable based on luck of the draw: how energy efficient the local cement plants, limestone quarries, gravel mines, etc are, how close they are to the building site and each other, and so on.
In any case, ICE reports a central value of 0.035 kg of carbon emissions per kg of concrete. Let's work with that for a minute.
Consider again the back-of-the-envelope house we were discussing the other day, which had interior dimensions of 45' x 29'. Let's suppose a fairly conventional kind of construction with a 4" slab on grade, and a 12" wide exterior footing wall that extends down 2'. That requires 27.35 cubic yards of concrete, which at a density of about 4000 lb/yard weighs around 50 metric tonnes. Thus the carbon emitted to produce that much concrete is 1.75 tonnes. A more complex plan, imperfections in site preparation, interior footings, etc, would probably raise the total by a few tens of percent.
This is substantially smaller than the 6.5 tonnes of carbon captured in straw bale walls. However, if the building department made us put in 24" wide footings, we would go up to 2.5 tonnes of emitted carbon. If we wanted a basement wall that thick, instead of a slab on grade, we would get up to 6.75 tonnes of emitted carbon. So we probably don't want to do it that way (and there's no reason to).
There are a variety of more creative alternatives, ranging from concrete blocks, rubble trench foundations, grade beams, earthbags, tires with rammed earth in, etc.
We are still neglecting lots of important things in this building (rebar in the concrete, carbon captured in the lumber elements, embodied energy of plumbing/wiring, etc), but those will have to wait for another day.
Tuesday, August 10, 2010
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5 comments:
Stuart,
This, like the earlier post on straw-bale construction, is interesting and contains useful data. However, I continue to think a focus on the embedded energy of construction is off target. A house will last 50, maybe 100 years or more. A typical house might emit 4 tons of carbon per year due to heating and electricity use. Say you lower that to 3 tons per year with materials that help conserve energy but are energy-intensive to produce, so you up the embedded energy from 1 ton to 10. After ten years your carbon emission will be lower. The ratio of embedded to operating energy will get progressively better as the years go by. Far more important than embedded energy are a building's durability and operating energy efficiency.
Mike:
I pretty much agree, but I intend to explore both aspects. My current hope is it's now practical to be carbon negative on both an initial and an operating basis (as long as one is willing to credit the carbon capture in timber/straw).
building code for a garage in my rural ohio county is 3 foot deep 8" concrete footers...
Suart
Rubble trench foundations are fantastic. See Fine Homebuilding number 18.
I have used them on all my buildings here, heavy adobe structures, no shifting or movement after 20 years.
You eliminate most of the foundation, substitute 3/4 gravel instead, and build on a 10 or 12 inch high "grade beam" which can be as wide as it needs to be.
Lots of dollar savings, and huge reduction in cement demand.
Also, with straw bale it's critical to get the moisture control details right, both for rain/snow, and for interior moisture bath/kitchen.
Straw bale is a wonderful material--but also something of a laborious, even pain in the ass material to work with.
RUdall
RUdall - Is that Randy Udall? If so, hi and great to hear from you!
On the pain-in-the-ass factor, I hear you. The other possibility I'm thinking about is agriboard which is basically and industrial SIP made from straw. It's clearly a more scalable possibility (in the sense that it's an industrial product in a way that straw bale will never be). However, we love the aesthetics of straw bale, and the Agriboard doesn't have the same amount of carbon capture or insulation value - it's basically half as thick.
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